Experimental investigation on sliding bubble coalescence of subcooled flow boiling in rectangular narrow channel

•Flow boiling bubble dynamic experiment is carried out with transparent heating wall.•The oscillation after bubble coalescence is divided into violent and moderation region.•A model is proposed to predict the relative velocity at the moment of bubble collision. The behavior of bubble coalescence in subcooled boiling has a significant influence on the void fraction and heat transfer capacity. In this paper, a visual flow boiling experiment is carried out to investigate the phenomenon of bubble coalescence and the effect of sliding bubbles on coalescence. The phenomenon of bubble coalescence, the velocity of bubble and interface evolution characteristics after coalescence are analyzed in detail. The annular waves caused by the rupture of the thick liquid film and the severe deformation caused by bubble coalescence are observed during the experiment. The propagation of annular waves on bubble surfaces makes bubbles unstable and then easily breaks down to produce secondary bubbles. Unlike the secondary bubbles produced by air bubble coalescence, the critical diameter of initial average bubbles that produce secondary bubbles after bubble coalescence is about 0.87 mm. This difference is because the viscosity of the fluid at high temperature in boiling two-phase flow is different from that in adiabatic water-air two-phase flow. Besides, it is found that there is an oscillation adjustment zone after bubble coalescence. This stage is divided into the violent oscillation region and moderation region, and their duration is basically the same. The relative velocity of sliding bubbles during collision is obtained according to the velocity difference between bubbles and the growth rate of the sliding bubble. Also, the effect of sliding bubbles on bubble coalescence is analyzed in this paper.